58 CEYLON PEARL OYSTER REPORT. 



The form of the anal process is likely to prove of diagnostic importance in the 

 characterization of species when a revision of the genus Margaritifera and allied 

 forms is undertaken upon the basis of a study of the soft parts. The variations in 

 the shells of M. vulgaris, which our plates exhibit, show that such diameters alone 

 are insufficient for an accurate differentiation of some species. 



The one living M. margaritifera, Linn. ("Black-lip"), which we had an oppor- 

 tunity of examining in Ceylon had an anal process wholly different from that of 

 M. vulgaris, being pinnatifid palmate, with five lobes (Plate III., fig. 5). 



In a third sjiecies, small and much flattened, which appears to be closely related to 

 the Shark's Bay shell, this process is simple, as in M. vulgaris, but is broader in 

 proportion, and with a rounded obtuse free termination, whereas in the latter it is 

 longer and more lanceolate, with an acuminate tip (see Plate III., figs. 1 and 3). 

 But probable variations in the soft parts must also be considered. 



The histology of the wall of the alimentary canal is shown in figs. 8, 9, and 10, 

 on Plate IX. The tube is lined by ciliated columnar epithelium throughout, and 

 presents no features that call for special detailed description. The columnar cells on 

 some parts of the wall are enormously taller than on other parts (see fig. 9), so as to 

 form great pads projecting into the lumen. The ciliated epithelium of the stomach 

 is continued for some distance along the larger ducts of the digestive gland (fig. 2). 



The caeca of the digestive gland are seen cut in various directions in figs. 1 and 2 

 on Plate IX. In a transverse section the exterior of the caecum is circular, but the 

 lumen is frequently quadrangular (fig. 4) because of the unequal size of the gland 

 cells. The organ agrees in detailed structure (see Plate IX., figs. 1 to 4) with that 

 of other better-known Lamellibranchs, such as the common oyster. 



THE BRANCHLE. 



Between the slightly opened valves of a living pearl oyster the four sickle-shaped 

 branch ire or gills, delicately fluted and usually more or less edged and dappled with 

 shades of grey and drab, are easily seen (Plate II., fig. 5). The free edges curve 

 outwards from the base of the foot (Plate II., figs. 3 and 4), and, keeping a little 

 inside of and parallel with the mantle edge, extend to a point just ventral to the 

 exhalent orifice, where they narrow to a well-defined combined tip (fig. 3). 



Of these four gills, two belong to each side of the body (Br., Plate V., fig. 3), and 

 each such lateral pair (Plate VIII., fig. 6) constitutes morphologically one ctenidium 

 so that one fold or " gill " is a hemi-ctenidium. The ctenidium consists of a 

 vascular basis or axis upon which are inserted at right angles along its whole length 

 two rows of long delicate branchial filaments, hollow outgrowths of the axis. As the 

 axis extends from the ventral border of the palps anteriorly along the front edge of 

 the visceral mass, nephridium and adductor muscle, and curves round ventrally and 

 posteriorly to a point opposite the anus, with its convexity first forwards and then 



